Introduction: Using a 4x4 KeyPad With CircuitPython
This inexpensive keypad offers a simple method of numeric input to your CircuitPython projects. I'm going to use it with an Adafruit ItsyBitsy M0 express.
You will need:
- Keypad - mine is 4x4
- ItsyBitsy M0 Express or similar board
- Mu Editor installed on your computer
- USB cable to flash the code
Step 1: How the Keypad Works
They Keypad has 16 simple, slow acting bubble switches arranged in a 4 by grid with 8 connections at the bottom. If we number from the left from 0 to 7, the connections 0 to 3 are connected to each of the rows. Connection 0 to the top row and connection 3 to the bottom row. Connections 4 to 7 are connected to the columns with connection 4 on the left column and connection 7 to the right column. Each of the 16 switches makes a join between a different row/column combination. If we press key7 the third row is joined to the left column. We can sense if the 5 key if being pressed if its row is raised HIGH and we can read a HIGH on its column. To do this we OUTPUT to the rows and INPUT from the columns.
We need to set each row HIGH in turn, while the other rows are LOW, read each column in turn until we get a HIGH input. This is easily managed with nested for loops.
We need to take into account of the fact that these bubble switches are quite slow acting and need to be pressed quite hard to make them close. Scanning all 16 switches in this way can be done very quickly but we have to scan repeatedly to pick up a key press. We also need to 'debounce' with a short time delay so that we do not get keys repeating each time we press.
As the switches are quite 'squishy' we need some feedback to the user to indicate that a key press has been read. The built-in LED is flashed each time a key press is sensed.
Step 2: Physical Connections
Connections left to right on KeyPad
0 1 2 3 4 5 6 7
D7 D9 D10 D11 A5 A4 A3 A2 on ItsyBitsy
D7 to D11 are set out as OUTPUTs while A5 to A2 are set up as INPUTs with PULLDOWN.
The code has plenty of comments and should be quite easy to follow.
Step 3: How Getkey() Works
This function scans the keypad for a single key press. It quickly checks each of the keys in turn a maximum of 10 times to see if a key switch is closed. If a key is pressed it returns the key value, counting left to right from top to bottom (0 ...15) Note the values for the bottom row: 14, 0, 15,13 (Hex E,0,F,D). If no key was pressed it returns -999 as an error code which can be easily picked up in the main program. The built-in LED flashes if a key press is picked up as user feedback. This runs pretty quickly and the de-bounce wait of 0.2 seconds is only used when a key is pressed.
Step 4: Getvalue(digits)
This routine supplies an n digit value from the numeric keys. It ignores the red keys.
This Python code can easily be converted to run on another device, such as a Raspberry Pi, you probably only need to change the pin setup lines at the top of the script.
Please let me know if you find the code useful.
Stay safe and have fun!